The project is to study the effect of sterol substitution in the membranes of a protozoan, Tetrahymena pyriformis, on the activity of certain membrane-bound enzymes, the fatty acid desaturases. It is hypothesized that the sterol content of the endoplasmic reticulum of these cells participates in regulation of the fatty acid biosynthesis process, to help govern the eventual fatty acid content (and therefore the fluidity) of the cellular membranes. The results may also help to explain early perturbation of cell physiology resulting from pathogenic accumulation of sterol (such as atherosclerosis).